Gender Construction
There are two different theories of gender: one says that our gender is related to our biological sex through chromosomes and hormones, the other says that gender is something we learn from the way we are socialised. In gender, it is accepted that gender is a social construction which may or may not relate to our biological sex.
A Conversation with Anne Fausto-Sterling (who specialises in the study of Intersex babies) suggests that there is spectrum of sex as well as a spectrum of gender. Her book Sexing the Body investigates many different aspects of biological sex (including the assumption of binary oppositions inherent in most apparently-objective research into genetics and endocrinology) and is available at the State library, UNSW library and Macquarie University library. I would use her bibliography and footnotes to follow up points you find especially interesting (and maybe come back to this book after your primary research to help with interpretation). Read a review of this book here. [If this becomes a key text, let me know and I'll buy a copy for the library.] For a critical perspective, How common is intersex? a response to Anne Fausto-Sterling argues against Anne's Fausto-Sterling's definitions of Intersex.
A Conversation with Anne Fausto-Sterling (who specialises in the study of Intersex babies) suggests that there is spectrum of sex as well as a spectrum of gender. Her book Sexing the Body investigates many different aspects of biological sex (including the assumption of binary oppositions inherent in most apparently-objective research into genetics and endocrinology) and is available at the State library, UNSW library and Macquarie University library. I would use her bibliography and footnotes to follow up points you find especially interesting (and maybe come back to this book after your primary research to help with interpretation). Read a review of this book here. [If this becomes a key text, let me know and I'll buy a copy for the library.] For a critical perspective, How common is intersex? a response to Anne Fausto-Sterling argues against Anne's Fausto-Sterling's definitions of Intersex.
Gender constructivism
One of the most powerful arguments against biological gender is the fact that parents treat their sexed children differently from the moment they are born, so there are socio-cultural influences constructing babies' identities from well before they have an understanding of themselves as individuals.
Multimodal Sex-Related Differences in Infant and in Infant-Directed Maternal Behaviors During Months Three Through Twelve of Development identifies ways in which mothers interact (esp. physically) with their babies based on their sex.
An interesting study in constructed gender is bacha posh in Afghanistan. These girls are not (usually) transgender. If a family has no boys, the parents may choose to bring up one of the girls as a boy from birth. These boys are required to transition back to being girls when they hit puberty and must quickly learn how to be female so that they can attain a 'good' marriage. The Underground Girls of Kabul (958.1044 NOR) is a journalistic account of these practices and the eventual effect on the bacha posh.
The Third Gender
Many societies include a third gender, often with a shamanistic or spiritual application. this interactive map of World Gender Customs idenitifies some of the these cultures (Note that the existence of an accepted gender classification, doesn't mean there is no discrimination against these genders). However, Gender as a historical kind: a tale of two genders? states that "both political movement[s] and social science should respect the particular cultural niche in which gender is reproduced and should be cautious in making sweeping cross-temporal and cross-cultural claims." You should take this ethnocentric globalising of trans identities into account when doing your research.
Not man, not women: Psycho-spiritual characteristics of a Western third gender explains that "there may be elements of third-gender-type difference that are transcultural and inherent to gender intermediacy. ... [and] that transpersonal theories may profit from the inclusion of an expanded view of gender not only to promote looking beyond deeply ingrained cultural assumptions about the inevitability of male/female duality and masculine and feminine norms, but also as a way of envisioning the healing of the mind/body split, following the example of those who live such reunification through the embodiment of (gender) paradox. "
Not man, not women: Psycho-spiritual characteristics of a Western third gender explains that "there may be elements of third-gender-type difference that are transcultural and inherent to gender intermediacy. ... [and] that transpersonal theories may profit from the inclusion of an expanded view of gender not only to promote looking beyond deeply ingrained cultural assumptions about the inevitability of male/female duality and masculine and feminine norms, but also as a way of envisioning the healing of the mind/body split, following the example of those who live such reunification through the embodiment of (gender) paradox. "
Fa'afafine (Samoa)
Hijra (South Asia)
Two Spirit (Zuni - Indigenous American)
Although around 150 different Indigenous American cultures understood some people as belonging to a third gender: "Before colonization, we were balance-keepers. We were the only ones that could move between the men’s and women’s camps." says Tony Enos in Native American Two-Spirits Look to Reclaim Lost Heritage. C19th anthropologists made the Zuni tribe the most famous. The "Middle" Gender in Zuni Religion and Rethnking Gender and Sexuality: Case Study of the Native American Two spirit People (which has an interesting comment about cultural appropriation of Two spirit concepts) describes these practices.
Sekrata (Madagascar)